Transport properties in electrically conductive polymeric materials

Abstract

Measurements on free standing films of the conductive polymer polyaniline (PANi) have revealed that charge transport within this material depends upon the level of intermolecular order. This factor is found to depend upon the method of sample preparation. PANi protonated by immersion of solid emeraldine base in aqueous methane sulphonic acid has low conductivity, 30-40 Scm(^-1). This can be enhanced, up to 250 Scm(^-1) if films are stretch oriented prior to protonation. Stretched samples have an electrical conductivity anisotropy factor of order 7 at 300 K, also revealed in their thermopower over the range 100 - 300 K. The behaviour of electrical conductivity with temperature is commensurate with charge transport in a disordered system. Protonation of PANi dissolved in meta cresol by addition of camphor sulphonic acid (CSA) yields material with conductivity of 250-300 Scm(^-1) Variation of the acid concentration has revealed a transition to a metallic response in conductivity (near 300 K) when 20-30% of polymer nitrogen sites are protonated. This character extends to progressively lower temperatures as protonation is increased to 60%. The metallic nature of this material is evident in the linear temperature dependence of thermopower and is ascribed to the presence of crystalhne regions within the polymer fihn, as revealed by an independent x-ray analysis The role of molecular order upon the properties of thin films of 3[2(S2-methylbutoxy)ethyl]-polythiophene has been investigated. Starting with polymer dissolved in 'good' solvent, quantities of nonsolvent lead to reorganisation of the sidechain groups when added. This promotes an increase in effective conjugation length which can be transferred to the solid state by the spin coating process as indicated by spectroscopic studies. With these films acting as the active layer in a field effect transistor the charge carrier mobility can be measured. It is found that as molecular order increases, mobility decreases from 10(^-5) cm(^2)V(^-1)s(^-1) to 710(^-8) cm(^2)V(^-1)s(^-1). This is ascribed to increased interchain separation and effects due to macroscopic aggregate grain boundaries.